stdenv: provide meta.evaluates

This gives a way to see the result of `check-meta` without triggering any assertions.

pkgs/stdenv/generic/check-meta.nix

@@ -159,6 +159,7 @@ let
     executables = listOf str;
     outputsToInstall = listOf str;
     position = str;
+    evaluates = bool;
     repositories = attrsOf str;
     isBuildPythonPackage = platforms;
     schedulingPriority = str;